Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Various techniques have been developed for obtaining digital X-ray and gamma ray images of an object for purposes such as X-ray diagnostics, medical radiology, non-destructive testing, and so on. Among different types of semiconductor-based imagers, a complementary metal-oxide-semiconductor (CMOS) imager includes a focal plane array of pixel cells, each one of the cells including either a photogate or a photodiode overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. A readout circuit is connected to each pixel cell and includes at least an output field effect transistor formed in the substrate and a charge transfer section formed on the substrate adjacent the photogate or photodiode having a sensing node, typically a floating diffusion region, connected to the gate of an output transistor. The CMOS imager may include at least one electronic device such as a transistor for transferring charge from the underlying portion of the substrate to the floating diffusion region and one device, also typically a transistor, for resetting the node to a predetermined charge level prior to charge transference.
In a CMOS imager, the active elements of a pixel cell perform the functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion region accompanied by charge amplification; (4) resetting the floating diffusion region to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. A photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion region. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower. The photosensitive element of a CMOS imager pixel is typically either a depleted p-n junction photodiode or a field induced depletion region beneath a photogate.
A conventional CMOS imager is susceptible to a type of distortion known as eclipsing. Eclipsing can occur when at least some pixels of the CMOS imager are exposed to strong light such as direct illumination from the sun. The strong light may cause electrons to spill over from the photodiode into the floating diffusion region, which results in an erroneous reset signal to be sampled (e.g., reset signals sampled during reset operations may exhibit voltage levels that are less than the desired reset level). Consequently, the pixel signal computed by a column readout circuitry becomes an undesirably small value, the effect of which is manifested when an over-illuminated pixel appears dark while it should be bright.
FIG. 1A is a schematic diagram of a pixel 100 and a source follower 250 for use in a conventional CMOS imager. The pixel 100 includes a light sensitive element PX (shown as a photodiode), a floating diffusion region FD, and four transistors M1˜M4. The source follower 250 includes an anti-eclipse circuit 35. The conventional anti-eclipse circuit 35 is configured to correct the voltage level of the reset signal by pulling the reset level up to a corrected voltage, thereby minimizing the eclipse effect. However, the actual clamp level of the anti-eclipse circuit 35 may drop with the increased load current ILOAD, thereby narrowing the dynamic range of the pixel voltage VPX (the difference between the sampled reset signal and the photo signal). The output swing of the pixel voltage VPX may lower the saturation of an image, such as a bright sun appearing to be a gray object.
It would therefore be desirable to provide a CMOS imager with an anti-eclipse circuitry for compensating errors caused by the eclipse phenomenon.